1. Field of the Invention
The present invention generally relates to piezoelectric polymers and more particularly to a method of inducing piezoelectric properties in polymers. The disclosed method provides a cost effective alternative to conventional electric field poling procedure of polymers. Specifically the present invention is a method of inducing piezoelectric properties in polymers at room temperatures using tensile poling methods.
2. Related Art
The piezoelectric effect in polymers such as polyvinylidene fluoride (PVDF) results from attaining specific directional dipole alignment along the polymer chain backbone. This is typically done by pre-aligning the polymer chains in a unidirectional manner, and then aligning the dipoles using an electric field. This later step is known as electric field poling. Electric field poling is a technique in which an electric field is applied to the polymer in a manner which orients the axes of crystallites in a preferred direction. Electric field poling of piezoelectric polymers is typically performed at elevated temperatures.
Much of the related art in this area has involved a method of inducing piezoelectric properties in PVDF films which included subjecting the PVDF films to stretching forces and then to a high electric field or corona charge at an elevated temperature to provide piezoelectric properties. Typically, such a method would yield piezoelectric constants in the range of approximately 30-35 pC/N, where C is Coulombs and N is Newton.
Another related method to induce piezoelectric properties in PVDF polymers is disclosed in U.S. Pat. No. 5,254,296. This disclosure reveals a method to increase the piezoelectric constant of PVDF films and co-polymers thereof up to about 60 pC/N when they are subjected to simultaneously stretching and corona poling with a poling field of about 0.55 MV/cm, a stretching ratio of about 4.5, and a poling temperature of approximately 80 degrees Celsius. Accordingly, most of the related art teaches methods of inducing piezoelectric properties in polymers at elevated poling temperatures.
It is believed that polyvinylidene fluoride (PVDF) films possess the highest values of piezoelectric constants of any known polymer. PVDF is semicrystalline and contains at least two stable forms, a polar form I containing beta-crystallites and a non-polar form II containing alpha-crystallites. The beta crystalline configuration has an extended all-trans polar conformation. The alpha crystalline configuration has a trans-gauche-trans-gauche non-polar conformation.
Efforts have been made to increase the dipole alignment by increasing the occurrences of the beta crystalline configuration within the polymer. Electric field poling at elevated temperatures converts the alpha crystalline configuration to the beta crystalline configuration and aligns dipoles giving rise to large polarizations. Increasing the beta crystalline configuration within the polymer has also been accomplished by incorporating temperature cycling procedures before and during the electric field poling. The conventional approach is to bring the temperature range of the polymer proximate to the glass transition temperature range. This is typically at or above 80 degrees Celsius. An electric field is then applied across the polymer which aligns the dipoles. The polymer is then quenched or otherwise cooled at a predetermined rate such that the polymer will stabilize with the new orientation and alignment of the dipoles.